It is desired that road surface conditions during vehicular travel be estimated with accuracy and the estimation be fed back to vehicular control to improve the running safety of vehicles. If road surface conditions can be estimated during vehicular travel, a more advanced control of ABS (antilock braking system) braking, for instance, can be realized before such danger avoidance action as braking, acceleration, or steering is taken. This is expected to further improve the safety of vehicular operation.
In a proposed method for estimating road surface conditions during vehicular travel, tire vibrations during vehicular travel, for example, are detected, and the vibrations in a time region before the leading edge of the contact patch (pre-leading-edge region) are extracted from a time-series waveform of the detected tire vibrations. At the same time, a vibration level ratio R, which is a ratio in magnitude of the vibration component in a high-frequency region (3 kHz to 5 kHz) of the vibrations to the vibration component in a low-frequency region (1 kHz to 2 kHz) thereof, is calculated. Then the road surface condition is estimated to be a high μ road surface or a low μ road surface, from the vibration level ratio R (see Patent Document 1, for instance).
Also, in another proposed method, sounds generated from a tire during vehicular travel are detected, and a mean value of the sound pressure level within a set frequency range of the tire-generated sounds is calculated. Then the mean value is compared with reference values so as to determine whether the road surface is an amply wet asphalt road surface, a slightly wet asphalt road surface, a dry asphalt road surface, or an icy road surface (see Patent Document 2, for instance). In still another proposed method, the ratios between the mean values of sound pressure levels within a plurality of frequency ranges are compared with reference values so as to determine whether the road surface is a snowy road surface, a watery road surface, an icy road surface, or a dry road surface (see Patent Document 3, for instance).